1. Field of the Invention
This invention relates to a method of making a bladed disc assembly of the kind used in the compressor turbine sections of a gas turbine engine.
2. Description of Related Art
An axial flow turbine of a typical gas turbine engine includes one or more bladed disc assemblies each comprising a central disc member mounted for rotation about the longitudinal axis of the engine. From the periphery of the disc of such an assembly there extends radially outwards a plurality of blades each of an aerofoil cross-section and forming a circumferential series around that periphery. A turbine may comprise several such bladed disc assemblies axially spaced along the axis of the engine, the axial spacing enabling a circumferential series of fixed guide vanes to be interposed between the blades of adjacent bladed disc assemblies each of the guide vanes being directed radially inwards from the casing which encloses the turbine.
The blades may be attached to the disc by means of fir tree root fittings on the blades, the root fittings being pressed into slots broached in the periphery of the disc and secured by peening on each side.
The blades of a bladed disc assembly may be made singly or in groups, and several methods are available for connecting blades to a disc.
In a typical example a single bladder, which may be cast from a nickel base alloy, is attached to a disc by means of a fir tree root on the blade, which root is pressed into a slot broached in the periphery of the disc. This form of attachment requires very accurate machining to ensure that the loading upon the blades is shared by all the serrations of the fir tree configuration.
In another example, groups of blades, eg three blades, are cast as a single unit and the appropriate number of such groups is assembled around the periphery of a disc before they are secured in place by fusion welding.
In yet another example a dual alloy disc is provided with a ring of cast turbine blades bonded to the discs.
Diffusion bonding which is essentially a solid-state bonding process, refers to the metallurgical joining of metal surfaces by the application of heat and pressure to cause intermingling of atoms at the joint interface. It differs from fusion welding in that fusion welding is dependent upon melting of the metals at the joint interface whereas, in most instances, diffusion bonding is accomplished entirely in the solid-state at a temperature well below the melting point of the metal.
Three major conditions require careful control for successful diffusion bonding, these being pressure, temperature and time (at a given temperature and pressure). The temperature accelerates the commingling of atoms at the joint interface and provides metal softening which aids surface deformation and more intimate contact. The time employed is controlled to be at a minimum, the time allowed being that sufficient to assume that surfaces are in intimate contact and some atomic movement has occurred across the interface. Generally, diffusion bonding is carried out in a vacuum or in an oxygen-free inert gas, eg argon, environment. It is essential to provide smooth clean conforming surfaces at the bond face when diffusion bonding, and in some cases, an intermediate layer of soft material such as molybdenum may be employed. The cleaning may for instance be provided by chemical cleaning methods.
Several techniques may be used in diffusion bonding, one such being hot isostatic pressure bonding where uniform application of pressure is necessary over all the surfaces of the joint. The level of pressure required is high. For example, diffusion bonding may be accomplished in an autoclave and the joint surfaces subjected to 15,000 psi (1054 kg./sq. cm.) of inert gas at over 500.degree. C., whilst in another example satisfactory bonding may require a pressure of 60,000 psi (4218 kg/sq. cm.) at 2000.degree. C., in which case the pressurising media may be an hydraulic fluid in combination with a material such as silica sand.